The seasonal movement of animals, especially of birds, fish and some mammals such as porpoises, is still not (11)................... understood. Climatic conditions are thought to trigger off migration where perhaps lower temperatures (12)................. in less food being available. Some animals, particularly (13) .................., travel vast distances. Golden Plovers are just one example, as they annually fly 8000 miles from the Arctic to South America. Migrating animals (14)................ to use three mechanisms for finding their way. Over short distances an animal moves to successive familiar landmarks and this is called piloting. In orientation, a straight line path is taken, (15)................. on the animal adopting a particular compass direction. Navigation is the most complex process as the animal must first (16)................. its present position before taking a direction relative to that. It seems that some birds (17).................. the sun, stars (often the North star which moves very little), and an ‘internal clock’ which (18)..................... allowances for the relative position of these heavenly (19).................. Even when the sun is hidden behind a cloud, many birds are (20)................... to continue their migration quiet accurately by plotting their direction with respect to the Earth’s magnetic field.